After several recommendations, I finally watched the movie Hard Candy. It’s quite an interesting thriller, although certainly not without flaws and plot holes. There’s some debate about whether the film is about child abuse, or about vigilantes. I’ll post some thoughts on that in the comments, to avoid spoiling details in case you haven’t watched the film. The visual style is cool, but a bit too "designed" for my taste – MTV/videoclip aesthetics. It works well in that it supports the dry atmosphere of the whole movie.
*SPOILERS AHEAD*
You could substitute the 14 yr old girl with a man of 25, and the child molestation themes with some other murder, and the entire plot would still work (plot holes aside). That said, our feelings for both characters would be less intense due to the horrific idea of children being the subject and perpetrators of such abuse.
Rather than vigilantism, I’d say it is about predators. I see her as seeking for pleasure in revenge, rather than justice. If she wanted justice, she would have exposed him and let prison-life do the rest; if she were just a vigilante, she would have simply killed him. Like a child molester, she will likely seek to satisfy that thirst for violent self-righteousness again in the future.